The present invention relates generally to semiconductor devices and methods of fabricating such devices and, more particularly, to semiconductor angled-facet laser devices (gain chips) having curved waveguides and mode transformers and methods of fabricating these devices.
Technologies associated with the communication of information have evolved rapidly over the last several decades. Over the last two decades, optical information communication technologies have evolved as the technology of choice for backbone information communication systems due to, among other things, their ability to provide large bandwidth, fast transmission speeds and high channel quality. Semiconductor laser devices are used in many aspects of optical communication systems, for example to generate optical carriers in optical transceivers and to generate optically amplified signals in optical amplifiers. In order to use laser devices in this capacity, they are coupled to the optical fibers over which the optical signals are carried.
There are various challenges that need to be addressed by the manufacturers of semiconductor laser devices in order to make these devices as efficient as possible for use in optical systems. For example, as seen in FIG. 1, an external cavity semiconductor laser comprises a semiconductor gain element 100 having front and back facets 102 and 104, respectively, with a waveguide 105 formed therebetween. A lens 106 (or other collimating optics) collimates the divergent optical output from waveguide 105 onto a diffraction grating 108. Diffracted light is then directed toward retroreflector 110, reflected back to diffraction grating 108 and, from there, back to front facet 102. This establishes a laser cavity between the retroreflector 110 and back facet 104. Optical energy is coupled outside the cavity from the semiconductor gain element 100 as shown by the dotted arrow in FIG. 1. Optical energy can also be extracted from the back facet through lens 112 as shown by the solid arrow in FIG. 1.
The high refractive index associated with the semiconductor material used to fabricate gain element 100 tends to cause reflections of the light from front facet 102 toward back facet 104. These reflections interfere with the primary lasing mode of the external lasing cavity and reduce the spectral quality of the laser emission. Initial attempts to reduce these undesirable reflections involved applying an anti-reflective coating to the front facet 102. However, these coatings are expensive and also are effective only in a relatively limited bandwidth. More recent innovations in reducing these unwanted reflections involve angling the facet 102 relative to the optical axis of propagation and providing a curved waveguide 105. These techniques are described, for example, in the article entitled “Single-angled-facet laser diode for widely tunable external cavity semiconductor lasers with high spectral purity”, to P. J. S. Heim et al., Electronics Letters, July 1997, Vol. 33, No. 16, pp.1387-1389, the disclosure of which is incorporated here by reference.
Another challenge faced by laser designers involves coupling the laser light generated by the laser of FIG. 1 to an optical fiber. Coupling of optical signals between a laser and an optical fiber results in coupling losses, which designers attempt to minimize in order to improve transmission and power efficiency. One cause of coupling losses is the different optical mode profiles (e.g., beam sizes and/or shapes) associated with lasers and optical fibers. Lasers typically have an elliptical mode profile whereas optical fibers typically have a circular mode profile that is also larger in dimension than the laser mode. Accordingly, it is desirable to transform the mode profile associated with the optical energy generated by the laser in the region where it is coupled to the optical fiber.
An example of a mode transformer for use in a semiconductor optical device is found in U.S. Pat. No. 5,278,926 to Pierre Doussiere, the disclosure of which is incorporated here by reference. Therein, the transverse cross-sectional area of an active waveguide decreases in a mode transition section to couple a narrow optical mode, which is generated and guided by the active waveguide, to a wide optical mode that is guided by a passive waveguide. However, the Doussiere patent describes a mode transformer employing active and passive waveguides having a linear geometry and is unconcerned with the implementation of a curved waveguide/mode transformer structure.
Accordingly, it would be desirable to provide semiconductor laser devices having curved waveguides and mode transformers that are optimized for implementation in curved waveguide devices, as well as efficient and cost effective methods of manufacturing such devices.